Some 200,000 residents near Lake Oroville, a California reservoir contained by the tallest dam in the United States, have been forced to evacuate after erosional damage created a hole in the dams spillway.

Eight cities downstream of the 770-foot dam were included in the evacuation order, issued around 5 p.m. local time on Sunday, Feb. 12, after it became clear that the integrity of the emergency spillway was severely compromised.

The emergency spillway has not been used since the dam was constructed in 1968.

Cracking in the earthen spillway began to appear last week and is said to be the result of unusual amounts of rainfall in the area.

“Officials now anticipate a failure of the auxiliary spillway within 60 minutes,” read a warning that was released by the National Weather Service on Sunday afternoon. “This is not a drill. This is not a drill. I repeat, this is not a drill.”

According to a local newspaper, three environmental groups—the Friends of the River, the Sierra Club and the South Yuba Citizens League—filed a motion with the Federal Energy Regulatory Commission back in 2005 warning them about the very scenario that unfolded Sunday night but no action was taken.

Officials now on the scene say that the dam itself is not currently in danger of a breach, but if it were to fail it could send a 30-foot tall wall of water down into the Feather River basin.

Watch a live stream of the events below.